Dispensing devices for dispensing a foam are known per se. U.S. Pat. No. 5,271,530 and U.S. Pat. No. 5,443,569, for example, disclose a dispensing device which comprises a pump assembly for forming a foam. The pump assembly comprises a liquid pump for pumping liquid and an air pump for pumping air to a common dispensing passage. The liquid pump and air pump can be actuated simultaneously by pressing a common operating button, the pumped liquid and air being mixed in a mixing chamber provided in the dispensing passage to form a foam, which foam is subsequently guided through a sieve element having two sieves for homogenizing and smoothing the foam. The formed foam is dispensed via a dispensing opening which is arranged in the common operating button.
The known dispensing device has proved to be very successful for forming and dispensing a foam with a large number of different applications, such as soap, shampoo, suntan lotion, dishwashing liquid, shaving foam, skin-care products and the like.
A drawback of the known dispensing device is that after the foam has been formed and dispensed by the operating element being pressed, a certain amount of foam remains behind in the dispensing passage. This foam will, possibly after it has become a liquid again, dry up.
Depending on the application for which the dispensing device is used and the liquid which is required for this purpose, this dried-up liquid will more or less become encrusted in the dispensing passage. This may be particularly disadvantageous with the sieves in the sieve element, since the dried-up and encrusted liquid may block the sieves and thus render it more difficult to subsequently dispense foam using the dispensing device or may even prevent it.
Another disadvantage of the known dispensing device is that the foam which remains behind in the dispensing passage, for example near the dispensing opening, can drip from the dispensing opening, in particular when the foam turns back into a liquid again. It is possible that this dripping occurs in particular when the dispensing device is moved or stored in a non-vertical position. This problem also occurs with dispensing devices which have been positioned or are operated in such a manner that the dispensing opening is at least partly pointing downwards, for example in a wall dispensing device which is arranged in a fixed position on the wall with the dispensing opening pointing downwards, such as are in use in public toilets. Such dripping is undesirable, in particular as it is possible that this dripping only occurs some time after the dispensing device has been used, that is when the foam has turned back into liquid.
It is known per se to allow the air, which is sucked into the air pump during the return stroke of the operating element for a new stroke, to flow through the dispensing passage so that this air sucks the foam back out of the dispensing passage and in particular out of the sieves. However, the foam is carried along into the air pump chamber and may there adversely affect the action of the air pump, as has been described above. Although such foam pumps are referred to as being self-cleaning, they do not achieve the desired result. The foam which remains behind in the dispensing passage is sucked back into the dispensing device, but may there turn into a liquid and still flow out of the dispensing opening.
In addition, the sucked-back foam and/or the liquid formed from it may dry up in the dispensing device and become encrusted and thus negatively affect the action of the dispensing device. In particular, in the known dispensing device, the foam/the liquid may end up on the air piston or in the air chamber of the air pump. Liquid which has dried up and become encrusted there may in particular reduce the guidance between the air cylinder and the air piston and thus the action of the air pump.